FIG. 1 depicts a schematic diagram of telecommunications system 100 in the prior art. System 100 comprises telephones 101 and 109, Public Switched Telephone Networks (PSTN) subnetworks 102 and 108, Internet Protocol (IP) gateways 103 and 107, Internet Protocol network 104, and Internet Protocol endpoints 105 and 106, interconnected as shown. System 100 enables telephones 101 and 109 and endpoints 105 and 106, as well as other telecommunications terminals, to communicate with each other various kinds of media such as audio, video, and so forth.
Each of telephones 101 and 109 is a telecommunications terminal that is capable of making calls to or receiving calls from any other telecommunications terminal—PSTN-based or IP-based—in telecommunications system 100.
Public Switched Telephone Network subnetworks 102 and 108 are portions of the Public Switched Telephone Network (PSTN). Subnetwork 102 comprises access paths, switches, and transmission paths, in a combination of analog and digital technology, which enable telephone 101 to communicate with other terminals. Subnetwork 108 also comprises access paths, switches, and transmission paths, in a combination of analog and digital technology, which enable telephone 109 to communicate with other terminals. Each depicted portion of the PSTN might comprise wireline equipment, wireless equipment, or both wireline and wireless equipment.
Internet Protocol gateways 103 and 107 are nodes that act as access points into Internet Protocol network 104 for signals from PSTN subnetworks 102 and 108, respectively.
Each of Internet Protocol endpoints 105 and 106 is a packet-capable telecommunications terminal that communicates via the Internet Protocol. Each endpoint is capable of making calls to or receiving calls from any other telecommunications terminal—PSTN-based or IP-based—in telecommunications system 100.
Internet Protocol network 104 is a packet-switched network that is capable of transporting packets from one node to another. The transported packets can comprise voice or video signal information in their payloads and can also comprise Real-time Transport Protocol (RTP) headers, User Datagram Protocol (UDP) headers, or IP headers. When the packets comprise voice signal information with IP headers, they are often referred to as Voice over Internet Protocol (VoIP) packets, and the networks that transport the VoIP packets are often referred to as VoIP networks. When each node in network 104 can be both a sender of packets and a receiver, there are k*(k−1) network paths through network 104, wherein k is positive integer that represents the number of nodes in the network.
The service provided by a network path in network 104 can be characterized by its “quality of service,” which, for the purposes of this specification, is defined as a function of the bandwidth, error rate, and latency from one node to another. For the purposes of this specification, the “bandwidth” from one node to another is defined as an indication of the amount of information per unit time that can be transported from the first node to the second. Typically, bandwidth is measured in bits or bytes per second. The bandwidth exhibited by the network can be compared to the bandwidth requirements of one or more media flows; the “bandwidth requirement” is the amount of information per unit time per media flow that has to be transported from the first node to the second, usually determined by the signal encoding protocol (e.g., G.711 for voice, etc.) that governs the particular media flow. For the purposes of this specification, the “error rate” from one node to another is defined as an indication of the amount of information that is corrupted as it travels from the first node to the second. Typically, error rate is measured in bit errors per number of bits transmitted or in packets lost per number of packets transmitted. For the purposes of this specification, the “latency” from one node to another is defined as an indication of how much time is required to transport information from one node to another, plus any packetization delays and buffering delays that accumulate at the endpoints. Typically, latency is measured in milliseconds. The quality of service provided by network 104 can vary based on the actual bandwidth, error rate, and latency experienced by the call or session that is being carried by network 104, in relation to the requirements for the bandwidth, error rate, and latency for the call or session.
The quality experienced in telecommunications system 100 can also depend on other factors. First, each of telephones 101 and 109 can influence audio call clarity through the quality of its loudspeaker and microphone, the loudness of the transmitted and received signal, and the acoustic echo generated between the loudspeaker and microphone. Second, where each of PSTN subnetworks 102 and 108 converts the analog voice signals from a telephone into digital signals to yield greater efficiency in the transmission backbone, digitizing those voice signals can affect the clarity. Third, each of gateways 103 and 107 can affect the clarity through its components such as speech codecs, silence suppression mechanisms, comfort noise generators, jitter buffers, and echo cancellers. And fourth, each of endpoints 105 and 106 also can affect the clarity through its components such as a speech codec, a silence suppression mechanism, and the quality of its loudspeaker and microphone. Many of the impairments that are presently experienced by telecommunications users are as the result of different networks, such as Voice over Internet Protocol networks versus the PSTN, having to interoperate with each other, where some of those networks—or at least the commercial application of those networks—are relatively new in telecommunications. Both the differences between the networks and the equipment that is necessary to enable the different networks to interoperate, such as gateways, are some of the causes of impairments, many of which were either imperceptible or nonexistent in a PSTN-only telecommunications environment.
Other configurations of “hybrid” telecommunications systems that comprise both PSTN and IP-based networks also exist in the prior art. In those other systems, as in the telecommunications system described above and with respect to FIG. 1, both the PSTN and the gateways that bridge the PSTN and IP-based networks can be sources of impairments.